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CBSE Class 11th Chemistry Notes for Chapter 3 Classification Of Elements And Periodicity In Properties

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    UNIT 3 CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES
 
The s-block elements
The elements of Group 1 (alkali metals) and Group 2 (alkaline earth metals) which have ns1 and ns2 outermost electronic configuration belong to the s-block elements. These are reactive metals with low ionization enthalpies.
The p-block elements
The p-block elements comprise of elements belonging to Group 13 and 18. These together with the s-block elements are called the Representative Elements or Main Group Elements.
The d-block elements
These are the elements of group 3 to 12 in the centre of the periodic table. These are characterised by the filling of inner d-orbitals by electrons and are therefore referred to as d-block elements. These metals form a bridge between the chemically active metals of s-block elements and the less active elements of group 13 and 14 and thus are called transition elements.
The f-block elements
The two rows of elements at the bottom of the periodic table are called the Lanthanoids and Actinoids. These two series of elements are called the inner transition elements.
 
 
TRENDS IN PROPERTIES
 
Atomic radius: The atomic size generally decreases across a period and increases down a group.
Ionic radius: The ionic radius decreases across a period and increases down a group.
Ionization enthalpy: The quantitative measure of the tendency of an element to lose electron is given its ionization enthalpy. The first ionization enthalpy generally increases as we go across and decreases as we descend into a group.
Electronic gain enthalpy: When an electron is added to a neutral gaseous atom to convert it into a negative ion, the enthalpy change accompanying the process is defined as the electron gain enthalpy. Electron gain enthalpy becomes more negative as we move across a period and more negative as we move down a group.
Electronegativity: The ability of an atom in a chemical compound to attract shared electrons to itself is called electronegativity. It generally increases across a period from left to right and decreases down a group. 

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